1. Field of the Invention
This invention relates to catheter systems for intravascular ultrasonic imaging, and more particularly, to a catheter having an acoustically driven turbine that rotates an ultrasonic beam continuously through an area of interest within a vascular vessel.
2. Description of Related Art
Ultrasonic imaging systems are increasingly desirable for use in catheter-based probes to produce high definition images of internal surface characteristics of a blood vessel of a human body. Catheter-based probes typically comprise a flexible tubular element that is adapted to be inserted into a blood vessel in the vascular system. For example, such a probe may be inserted into the femoral artery of a patient in order to examine the coronary vessels and identify any stenosis or occlusion present within the vessels. This procedure can avoid the need for more invasive diagnostic techniques which might increase the risk to the patient and the associated recovery time. Catheter-based ultrasonic imaging is also known to improve the effectiveness of interventional therapies, such as angioplasty, atherectomy, laser ablation, and drug delivery, by enabling such therapies to be specifically directed where they will be most effective, and to evaluate the outcome of such therapy.
The ultrasonic imaging systems of these catheters typically comprise a piezoelectric transducer that generates an ultrasonic signal in response to an external electrical excitation. The ultrasonic signal is directed to an area of interest within a vessel, where it propagates through the blood until it reaches the interior surface of the vessel. Reflections of the signal, or echoes, return to the piezoelectric transducer, which converts these echoes to corresponding electrical signals. The electrical signals are then collected, processed and displayed as a two-dimensional image on a CRT screen.
In order to obtain a complete image of the interior surface area of the vessel, it is necessary to sweep the ultrasonic signal in a pattern about an axis of the vessel. Various techniques have been proposed to sweep the ultrasonic signal in the desired pattern which include the placement of the piezoelectric transducer in the distal end of the catheter. The transducer may be rotated directly to sweep the ultrasonic signal in the desired pattern, or the transducer may be fixed within the catheter and a reflective surface rotated to sweep the ultrasonic signal in the desired pattern. See U.S. Pat. No. 5,000,185, issued to Yock, for METHOD FOR INTRAVASCULAR TWO-DIMENSIONAL ULTRASONOGRAPHY AND RECANALIZATION. In this reference, torque for the rotation is provided by an external motor connected through the catheter by a torque cable to either the transducer or the reflective surface. Alternatively, a fluid coupled turbine may be disposed in the distal end of the catheter to provide the rotational torque. See U.S. Pat. No. 5,271,402, issued to Yeung et al., for TURBINE DRIVE MECHANISM FOR STEERING ULTRASOUND SIGNALS. In yet another alternative technique, a micro-motor may be disposed in the distal end of the catheter to provide the rotational torque. See U.S. Pat. No. 5,176,141, issued to Bom et al., for DISPOSABLE INTRA-LUMINAL ULTRASONIC INSTRUMENT.
There are numerous disadvantages associated with placing the piezoelectric transducer in the distal end of the catheter. The transducer may emit leakage currents inside the patient that can induce fibrillation when the probe images a coronary artery. Electrical wires that connect the transducer to external circuitry inherently act as antennas and receive radio frequency (RF) interference present within the environment of the catheterization laboratory. This RF interference may appear as noise in the electrical signals travelling to and from the transducer which distorts the two-dimensional image.
Another disadvantage of placing the transducer at the distal end of the catheter is that it increases the difficulty of varying the frequency of the ultrasonic signal. The piezoelectric transducer has a frequency of operation determined by its thickness. It may be desirable for the probe operator to adjust the transducer frequency in order to obtain a more precise image resolution or to illuminate a particular region of interest within the vessel. The transducer thickness is limited by the rather confined space within the distal end of the catheter, and the transducer cannot be easily replaced during catheterization.
Yet another disadvantage of placing the transducer at the distal end of the catheter is its associated expense. The catheter is typically discarded after a single use in order to prevent the transmission of disease. The transducer is costly to manufacture, and its disposal increases the already high cost of probe-catheterization techniques. Moreover, the higher resolution transducers are among the most expensive to manufacture, which tends to discourage use of such transducers in favor of less desirable imaging systems.
An alternative approach to these prior art techniques is to dispose the piezoelectric transducer external to the patient, and to direct the ultrasonic signal into the catheter by use of an acoustic waveguide. To sweep the ultrasonic signal in the desired pattern, the entire acoustic waveguide is rotated by an external device, such as a motor. See U.S. Pat. No. 5,284,148, issued to Dias et al., for INTRACAVITY ULTRASOUND DIAGNOSTIC PROBE USING FIBER ACOUSTIC WAVEGUIDES. This approach substantially minimizes the disposable elements of the catheter, and provides greater flexibility to the operator in terms of transducer selection.
Nevertheless, spinning waveguide imaging systems have substantial drawbacks as well. First of all, rotation of the acoustic waveguide subjects the relatively delicate waveguide to undesirable mechanical stress which could potentially damage the waveguide or reduce its acoustic conductivity. Second, the spinning waveguide is susceptible to non-uniform rotational velocity due to frictional binding of the waveguide by contact with the catheter sidewall. The frictional binding is generally most severe when the probe operator is attempting to steer the catheter through a sharp turn, such as from the aorta to the entry into the coronary vessel, or other so-called "torturous turns" within the vascular system. At the turning point, the spinning waveguide is pinched slightly which causes the overall rotational rate of the waveguide to decrease momentarily. The decrease in rotational rate may also be followed by a slight increase in the rotational rate due to an energy release caused by the sudden un-binding of the waveguide. The fluctuations of the rotational rate of the spinning waveguide can result in undesirable distortion of the two-dimensional image. The spinning torque cable imaging catheters described above are also prone to such non-rotational velocity effects.
Accordingly, it would be desirable to provide a catheter-based ultrasonic imaging system capable of sweeping an ultrasonic signal in a pattern about an axis of the blood vessel, which overcomes the numerous disadvantages of the prior art.